1. Field of the Invention
The teachings herein relate to limiting power density induced by a laser and a corresponding temperature increase in a sample interrogated by the laser.
The invention generally relates to spectral analysis systems, more particularly, the invention particularly relates to improvements in Raman systems to permit effective and rapid sample identification.
2. Description of the Related Art
Due to the strong optical absorption in some solids, most of the signal needed to perform a spectroscopic identification is absorbed by the sample and thus unavailable for detection. At the same time, absorption may also lead to a significant thermal change such as a rapid heating, melting and even burning of the sample during the identification process. Absorption may also lead to detonation of some explosive samples.
Thus, not only are the signal levels from the samples very small, but also these weak signals, particularly Raman signals, may be further obscured by large interfering backgrounds due to the fluorescence from thermally induced changes in the sample.
For example, white plastics can be easily and rapidly identified in 0.1 seconds with a Raman spectrometer, such as that disclosed in International Publication WO 99/01750, using a 1 Watt diode laser power, while black plastics cannot be identified under the same conditions due to laser induced detrimental changes.
In order to avoid laser induced detrimental changes in the plastic, it is necessary to decrease the laser power density on the surface of the sample. One way to reduce laser power density is to reduce total laser power that illuminates the surface of the black plastic. But at the same time, to accumulate enough signal for identification the signal collection time has to be increased proportionally. Obviously, this is not acceptable for rapid identification.
Another way to reduce the power density of the laser beam is to increase the size of the laser spot that illuminates the surface of the plastic, while still maintaining a sufficiently high laser power of 1 Watt to allow rapid identification. Experiments have shown that to avoid laser induced detrimental changes in black plastic samples, in the case of 1 Watt total laser power at wavelength 800 nm, the size of the laser spot illuminating the surface of a black plastic sample needs to be increased 40 times, to a size that is greater than 3 mm in diameter to avoid adverse impact on the sample. As a consequence, the signal acceptance area of the collection fiber bundle and the acceptance area of the spectrograph (slit-height times slit-width) must also be increased 40 times.
It will be appreciated that increasing the signal acceptance area of a collection fiber bundle by a factor of 40 is difficult, if not impossible, to achieve from a technical point of view. Enlarging the laser spot size without changing the optical train and components would cause the signal from the sample to overfill the collection fiber bundle and thus decrease the collected signal intensity.
Thus, there exists a need for a quick yet effective method to identify materials using spectral analysis, particularly Raman spectroscopy without damaging the samples.